Optical submarine systems usually comprise cables which contain repeaters spaced at suitable intervals, e.g. about 50 km. Each cable contains several, e.g. 6 or 8 optical fibres which transmit the telecommunications traffic. In addition to the fibres, the cable usually contains a conductor to provide electrical power to the repeaters and strength elements, e.g. tensile wires, to increase its mechanical strength and to protect the fibres. Usually the tensile wires are in contact with the conductor so that the wires can assist in carrying the electrical current. The whole of the structure is enclosed in a waterproof sheath, usually polyethylene, which also provides electrical insulation. The cable usually has an annular structure with the fibres at the centre surrounded by the electrical conductor and the tensile elements and with the sheath on the outside. To give some idea of the dimensions, a typical cable has an overall diameter of 25 mm, the sheath is 5 mm thick, and the centre core, which contains all of the fibres, is usually about 2 mm thick. In shallow water, where cables are liable to be damaged by maritime operations such as fishing and dropping anchors, the structure described above may be contained inside armour.
The repeaters are needed because fibres attenuate signals whereby amplification is required at suitable distances. This invention is particularly concerned with repeaters in which the amplification is provided by a fibre amplifier. A fibre amplifier usually comprises a suitable length of fibre, e.g. 1-20 m, which contains a lasing additive such as a rare earth element. The fibre amplifier comprises a pump, e.g. a laser operating at 1480 nm, which produces a population inversion in the energy states of the lasing additive, whereby optical signals are amplified by laser action. The fibre amplifier usually includes an automatic gain control device (AGC) which monitors the strength of the amplified signals. The amplifier includes control means which adjust the power in the pump laser to maintain a constant signal level at the output. One method of improving the performance of the AGC comprises providing a control tone on the optical signals. The AGC detects the control tone and maintains its amplitude at a constant value. This technique guards against optical noise, e.g. from pumps, affecting the performance of the AGC.
It is possible that the optical cables described above may get damaged and, therefore, it is desirable to provide the system with a default mode which is adopted when the fibre is damaged. Clearly the breakage of a fibre means that no signals are transmitted through the break in the fibre and the amplifiers after the break receive no input. This implies that there is no amplified output or that the amplified output falls below a threshold level. Where a control tone is used the control tone falls below a threshold level. When low or no output is detected the system adopts a default condition. It should be noted that, because the conductor and the fibres represent a small filament in the centre of the cable, if any element is damaged it is usual that all the elements are damaged. Thus, although breakages are themselves unusual, when a breakage does occur, it usually affects all the systems of the cable. EP-0,331,304, (British Telecommunications) and its corresponding counterpart U.S. Pat. No. 4,995,100 describe a means of using an AGC (Automatic Gain Control) to amplify signals and also to detect the existence of breaks in cables. In that specification the AGC circuit responds to a control tone transmitted at a different frequency to the data signals. If the control tone drops below a predetermined threshold level the repeater then switches into distress mode. This will indicate either a break in the optical fibre or a failure of an amplifier.
JP60-177238 (Mitsubishi Electric Corp) also describes a method of transmitting a control tone at a different frequency to the data tones. The levels of the control tone and the data signal are compared by comparator means at the receiving station. Breakage of the fibre is detected by drop in intensity of the received signals.
When a cable breaks, conventional default systems, as will be described in greater detail below, are able to identify the repeater adjacent the break. However when repeaters are spaced at substantial distances, e.g. 50 km or more, recovery and repair work may be prolonged by the need to conduct marine operations to locate the break if it occurs between repeaters. It would facilitate marine operations if the distance from the repeater to the break could also be established. Because maritime recovery and repair operations involve substantial lengths of cable, e.g. up to 5 or 10 km, great precision is not needed, and it would be satisfactory if the break could be located to the nearest kilometer (i.e. an error of .+-.0.5 km). This can be achieved by transmitting a light pulse from an adjacent repeater to the break. The break reflects the pulse back to the repeater and measuring the total time enables the distance of the break to be estimated. Techniques in which a pulse is transmitted into fibre and returned signals are measured and timed are known by the generic name of Optical Time Domain Reflectometry which is conveniently shortened to OTDR. It has been proposed to use OTDR in repeaters in order to locate breaks. This invention relates to the application of OTDR to repeaters which contain optical amplifiers and one of the objects of the invention is to simplify the hardware.